In conventionally-known mobile multi-hop networks such as DTN and ad hoc networks, paths from source nodes to destination nodes cannot be fixed due to movements of nodes so that they need to be timely optimized based on node allocations in each data transmit mode. Ad hoc networks having relatively dense distributions of nodes and easily forming direct end-to-end paths may generally adopt MANET (Mobile Adhoc NETwork) routing protocols such as AODV (Ad Hoc On Demand Distance Vector algorithm) and OLSR (Optimized Link State Routing protocol).
DTN refers to network systems guaranteeing highly reliable end-to-end transmission of data/contents in wireless ad hoc networks and satellite lines having unstable and low-reliable internode connectivity. In DTN, transmission data are subdivided into units of data sizes referred to as bundles, so that transmission processes toward destination nodes are performed in units of bundles. As the most significant feature of DTN, it is possible to point out a feature in which bundles are not discarded but accumulated until next hops of transmission are found in a standby state. According to this procedure, bundles are temporarily transmitted to intermediate nodes and thereafter retransmitted upon establishing paths toward destination nodes in networks having sparse distributions of nodes in which end-to-end paths from source nodes to destination nodes cannot be concurrently established, thus achieving results of improving transport factors toward destination nodes.
Therefore, the path control method of DTN adopts a different method compared to the conventional IP routing for searching end-to-end paths from source nodes to destination nodes.
One representative method regarding the DTN routing is disclosed in Non-Patent Document 1 and referred to as PROPHET which is a routing method based on delivery predictability toward destination nodes.
Referring to Non-Patent Document 1, the method referred to as PROPHET is explained.
PROPHET is designed to perform routing based on delivery predictability toward destination nodes. Specifically, when a certain node A adjoins another node B, the node A updates a delivery predictability P_i for the node B according to the following equation.P—i=P_(i−1)+(1−P_(i−1))*P_init(0<P_init<1)
where P_(i−1) denotes a delivery predictability of the node A for the node B, and P_init denotes an initialization constant.
As long as the situation in which the node A does not adjoin the node B remains, the delivery predictability is periodically updated and decreased by way of an aging method according to the following equation.P—i=P_(i−1)*γ^k(0<γ1)
where γ denotes the number of times each unit time has passed after the last time at which the node A adjoins the node B.
When each node has a bundle destined to a certain node, it exchanges the delivery predictability P_i for the destined node with an adjacent node, thus transferring the bundle to the adjacent node having the highest delivery predictability. When the delivery predictability of a certain local node for a destination node is lower than the delivery predictability of an adjacent node for the destination node, the local node does not transfer the bundle thereof to the adjacent node but retains it therein.
As described above, PROPHET is designed to perform routing through high/low comparison of delivery predictabilities for designation nodes calculated based on adjacency records.
Patent Document 1 discloses a method in which delivery predictabilities are calculated based on adjacency records whilst delivery predictabilities for destination nodes are calculated based on records of end-to-end paths for destination nodes, counted from the past to the present, which have been already registered with routing tables of nodes. As described above, it is possible to expand PROPHET such that delivery predictabilities embrace values calculated based on adjacency records as well as values calculated based on other parameters disclosed in Patent Document 1.